Biomedical Engineering Reference
In-Depth Information
performed through openings in the electrode holder. When closed electrodes are
used, a process of electrode detachment, scalp preparation, and electrode
reattachment is necessary and this is too time-consuming and error-prone to allow
for efficient, high-density EEG recordings. Accordingly, preparation time for high
electrode counts can be efficiently reduced by using open, ring-shaped electrodes
[e.g., as shown in Figure 2.3(a)] that do not require detachment for scalp
preparation.
The use of electrode caps and Ag/AgCl ring electrodes does not abolish the need
to prepare the scalp by slight mechanical abrasion to establish the impedance
between 5 and 10 k
that is required by traditional EEG amplifiers for operational
stability and noise immunity. With modern EEG amplifiers, homogeneity of elec-
trode impedances is more important for good common mode rejection than achiev-
ing low electrode impedances per se. Skin preparation, taken together with gel
application and impedance checking, still takes a substantial amount of time, but
because this is a critical step in achieving good EEG measurements, one should defi-
nitely properly and carefully prepare the scalp, because time saved here invariably
means more time can be devoted to data processing efforts. Figure 2.3 shows exam-
ples of passive electrodes commonly used today.
Several EEG equipment manufacturers have tried to overcome the limitations
imposed by the time-consuming scalp preparation. The most notable example of
this is the HydroCel Geodesic Sensor Net (GSN) by Electrical Geodesics Incorpo-
rated (EGI, Eugene, Oregon), shown in Figure 2.3(d). This electrode net system does
not consist of a traditional textile fabric cap, but instead of a geodesic (i.e., shortest
distance between two points on the surface of a sphere) arrangement of flexible rub-
ber-band-like fibers interconnecting the individual electrode holders. The electrode
holders themselves contain the electrode pellets. The HydroCel net can be used with
sponge inserts soaked in potassium chloride saline for recordings of up to 2 hours or
without sponge inserts but with electrolyte applied directly into the electrode wells
for recordings of longer than 2 hours. The GSN is then applied to the head much like
a wig and the individual electrode holders are straightened out to enable good place-
ment and electrode contact. The entire procedure can be performed in about 10 min-
utes for 128-channel nets, which makes this an attractive design for fast EEG
acquisition preparation. The sponge element holding the saline or electrolyte solu-
tion keeps the conductive electrode pellet in contact with the scalp and no further
impedance reduction is needed. The resulting scalp impedance typically is on the
order of 30 to 70 k
Ω
, which necessitates the use of a special amplifier with an
equally increased input impedance (of 200 k
Ω
or more). Also, as compelling as the
obvious time advantage of using this system is, it must be noted that this electrode
system and EEG amplifier may not always be perfectly suited for the optimization of
the EEG SNR due to the high impedance measurements employed.
Another variant of this scheme is the Quick Cell system made by
Compumedics-Neuroscan (El Paso, Texas), shown in Figure 2.3(c), which consists
of classical Ag/AgCl electrodes that can be fitted with cellulose sponges. After the
cap is placed on the head, a small amount of a special electrolyte solution is injected
into each electrode, thus wetting the sponge, which expands in response to the fluid
contact for recordings of up to 3 hours. If an impedance level of 30 to 50 kOhms is
deemed sufficient and a high input impedance EEG amplifier is used, then no further
Ω
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